“The only way to guarantee that nuclear weapons will never be used again is through their total elimination,” Kurz told the 191 parties to the treaty, the world’s benchmark arms control accord. “All states share the responsibility to prevent the use of nuclear weapons.”

Diplomats from the 159 countries supporting the ban, presented ahead of the 70th anniversary of the U.S. atom bombs dropped on Japan, said the initiative was modelled on successful campaigns to ban land mines and other weapons and could take years to move forward.

The initiative has virtually no support among NPT nuclear weapons states and veto-wielding Security Council members - the United States, Britain, France, Russia and China - or the countries of NATO, an alliance that provides a kind of “nuclear umbrella” security guarantee for its members.

But most of the 193 U.N. members back it.

The five permanent Security Council members signed the NPT as nuclear weapons states, although the pact calls on them to negotiate the reduction and eventual elimination of their arms caches. Non-nuclear states complain that there have been too few steps towards nuclear disarmament.

Four other states presumed to have nuclear weapons - Israel, Pakistan, India and North Korea - are not listed as supporters of the initiative.

Iran, accused by Western powers of developing a nuclear weapons capability under cover of a civilian programme, says its programme is peaceful. It is in talks with six world powers to curb sensitive nuclear work in exchange for sanctions relief. Tehran supports the Austrian initiative.

Without any explanation, Zarif, who on Monday spoke on behalf of the Non-Aligned Movement, cancelled a planned speech in his national capacity on Tuesday morning as news broke of Iran’s seizure of a cargo ship in the Gulf.

“Zarif decided that he did not have much to add to the NAM statement he gave on Monday. Hence it was decided not to give a national statement,” a diplomat at the conference told Reuters on condition of anonymity.